1. Field of the Invention
This invention relates to an optical head used for recording and/or reproducing an information recording medium, such as an optical disc, and a recording and/or reproducing apparatus provided with such optical head. This invention also relates to a recording and/or reproducing method for an information recording medium, such as an optical head, and to a method for detecting the thickness of a light transmitting layer of an information recording medium having a light transmitting layer formed on the recording medium.
2. Description of the Related Art
An information recording medium, such as read-only optical disc, a phase-change type optical disc, a magneto-optical disc or an optical card, is extensively used for storage of data, such as the video or audio information or computer program data. Recently, the demand for a high recording density and large recording capacity for these information recording mediums is becoming more and more imminent.
For raising the recording density of the information recording medium, it is effective to increase the numerical aperture NA of an objective lens loaded on an optical head and to shorten the wavelength of the light used to reduce the size of the laser light spot formed by the objective lens.
Thus, with a compact disc (CD) developed earlier as a digital optical disc, the numerical aperture NA of an objective lens is 0.45 and the wavelength of the light used is 780 nm. On the other hand, with a DVD, a digital optical disc higher in recording density and recording capacity than the compact disc, the numerical aperture NA of the objective lens is 0.6 and the wavelength of the light used is 650 nm.
Meanwhile, the information recording medium, such as an optical disc, has a light transmitting layer on a recording layer having the information signals recorded thereon. This recording layer is illuminated with a recording layer carrying the information signals through the light transmitting layer for recording or reproduction. If there is any error in the thickness of the light transmitting layer which deviates from a prescribed value, spherical aberration is produced due to this error. Of this spherical aberration, a degree-three spherical aberration, for example, is represented by the equation (1):
W40={xcex94t(n2xe2x88x921)/(8n3)}NA4xe2x80x83xe2x80x83(1)
where xcex94t is a thickness error of the light transmitting layer, n is the refractive index of the light transmitting layer and NA is the numerical aperture of an objective lens.
As may be seen from the equation (1), the spherical aberration attributable to the thickness error of the light transmitting layer is increased in proportion to a fourth power of the numerical aperture NA. Thus, if the numerical aperture in particular is increased, it is crucial to suppress the occurrence of the spherical aberration.
As may also be seen from the equation (1), it is effective to narrow the tolerance of the thickness of the light transmitting layer to reduce its thickness error for suppressing the address signal spherical aberration. For example, the thickness tolerance of the light transmitting layer in the DVD is xc2x10.03 mm. If the spherical aberration attributable to the thickness error of the light transmitting layer is to be suppressed to a value of the same order of magnitude as that of the DVD with the numerical aperture NA of 0.6, it suffices if the thickness error xcex94t of the light transmitting layer is within a range of the equation (2):
xe2x88x920.00388/NA4xe2x89xa6xcex94txe2x89xa6+0.00388/NA4xe2x80x83xe2x80x83(2).
From the above equation (2), it is possible to find the tolerance value required for suppressing the spherical aberration to substantially the same order of magnitude as that for the DVD in case of enlarging the numerical aperture NA. That is, it may be seen from the above equation (2) that, for the numerical aperture NA=0.7 or NA=0.85, it is sufficient if xe2x88x920.016 mmxe2x89xa6xcex94txe2x89xa6+0.016 mm or xe2x88x920.0074 mmxe2x89xa6xcex94txe2x89xa6+0.0074 mm, respectively.
However, it is extremely difficult to narrow the thickness tolerance of the light transmitting layer. Although it is not a significant process change for the mass-producing system, it is extremely difficult to raise the precision of the thickness error of the light transmitting layer because such error depends on the particular manufacturing method of the information recording medium. If the precision of the thickness error of the light transmitting layer could be achieved, significant precess changes and so forth are required, thus significantly increasing the manufacturing cost. It is therefore not advisable to narrow the thickness tolerance of the light transmitting layer to suppress the spherical aberration.
It is therefore an object of the present invention to provide an optical head and a recording and/or reproducing method and apparatus in which the spherical aberration can be suppressed even if the numerical aperture NA is increased.
It is another object of the present invention to provide a recording and/or reproducing method in which the spherical aberration can be suppressed even if the numerical aperture NA is increased.
It is yet another object of the present invention to provide a thickness detection method in which the thickness of a light transmitting layer formed on a recording layer of an information recording medium can be detected easily.
In one aspect, the present invention provides an optical head for an information recording medium having a light transmitting layer on a recording layer adapted for recording information signals, including a light source for radiating the light, an objective lens for converging the light from the light source via the light transmitting layer on the recording layer, an optical element of a pre-set refractive power arranged between the light source and the objective lens, and movement means for causing movement of the optical element responsive to the thickness of the light transmitting layer.
In the above optical head, the optical element is preferably a collimator lens. If the light transmitting layer is of a prescribed value, the collimator lens substantially collimates the light radiated from the light source to fall on the objective lens.
If, in the optical head, the numerical aperture NA of the objective lens is not less than 0.65, the light transmitting layer of the information recording medium preferably has a film thickness not less than 0.47 mm.
Preferably, the movement means includes a reference shaft arranged substantially parallel to the optical axis of the light radiated from the light source to on the optical element, optical element supporting means for supporting the optical element and adapted for being translated along the reference shaft, a motor, and a gearing for converting the motor rotation into a translating movement parallel to the optical axis and for transmitting the translating movement to the optical element supporting means. The motor rotation is converted by the gearing into the translating movement parallel to the optical axis to cause movement of the optical element supporting means to cause movement of the optical element so as to cancel the spherical aberration responsive to the thickness of the light transmitting layer.
With the present optical head, the optical element of a pre-set refractive power, arranged between the light source and the objective lens, is moved by movement means to cancel the spherical aberration depending on the thickness of the light transmitting layer. Thus, the spherical aberration ascribable to the error in thickness can be suppressed despite thickness error in the light transmitting layer.
In another aspect, the present invention provides a recording and/or reproducing apparatus for recording and/or reproducing information signals for a recording layer of an information recording medium also having a light transmitting layer on the recording medium, including thickness detection means for detecting the thickness of the light transmitting layer, and an optical head for an information recording medium having a light transmitting layer on a recording layer adapted for recording information signals. The optical head includes a light source for radiating the light, an objective lens for converging the light from the light source via the light transmitting layer on the recording layer, an optical element of a pre-set refractive power arranged between the light source and the objective lens, and movement means for causing movement of the optical element responsive to the thickness of the light transmitting layer as detected by the thickness detection means.
In the above optical head, the optical element is preferably a collimator lens. If the light transmitting layer is of a prescribed value, the collimator lens substantially collimates the light radiated from the light source to fall on the objective lens.
If, in the optical head, the numerical aperture NA of the objective lens is not less than 0.65, the light transmitting layer of the information recording medium preferably has a film thickness not less than 0.47 mm.
In the above recording and/or reproducing apparatus, the movement means includes a reference shaft arranged substantially parallel to the optical axis of the light radiated from the light source to fall on the optical element, optical element supporting means for supporting the optical element and adapted for being translated along the reference shaft, a motor and a gearing for converting the motor rotation into a translating movement parallel to the optical axis and for transmitting the translating movement to the optical element supporting means. The motor rotation is converted by the gearing into the translating movement parallel to the optical axis to cause movement of the optical element supporting means to cause movement of the optical element so as to cancel the spherical aberration responsive to the thickness of the light transmitting layer.
With the present recording and/or reproducing apparatus, the optical element of a pre-set refractive power, arranged between the light source and the objective lens, is moved by movement means to cancel the spherical aberration depending on the thickness of the light transmitting layer. Thus, the spherical aberration ascribable to the error in thickness can be suppressed despite thickness error in the light transmitting layer.
In a still another aspect, the present invention provides a recording and/or reproducing method for recording and/or reproducing information signals for a recording layer of an information recording medium also having a light transmitting layer, including using an optical head having a light source for radiating the light, an objective lens for converging the light from the light source via the light transmitting layer on the recording layer, and an optical element of a pre-set refractive power arranged between the light source and the objective lens, and detecting the thickness of the light transmitting layer to cause movement of the optical element responsive to the results of detection so as to cancel the spherical aberration.
In the above optical head, the optical element is preferably a collimator lens. If the light transmitting layer is of a prescribed value, the collimator lens substantially collimates the light radiated from the light source to fall on the objective lens.
If, in the optical head, the numerical aperture NA of the objective lens is not less than 0.65, the light transmitting layer of the information recording medium preferably has a film thickness not less than 0.47 mm.
With the present recording and/or reproducing apparatus, the optical element of a pre-set refractive power, arranged between the light source and the objective lens, is moved by movement means to cancel the spherical aberration depending on the thickness of the light transmitting layer. Thus, the spherical aberration ascribable to the error in thickness can be suppressed despite thickness error in the light transmitting layer.
In yet another aspect, the present invention provides a method for detecting the thickness of a light transmitting layer provided on a recording layer of an information recording medium, the recording layer being adapted for recording information signals thereon, including radiating light from a light source, converging the light radiated by the light source by an objective lens on the information recording medium, receiving the return light converged by the objective lens on the information recording medium and reflected from the information recording medium by a photodetector to detect focussing error signals, and detecting the thickness of the light transmitting layer from signal portions of the focussing error signals due to the return light reflected by the photodetector and those due to the return light reflected by the surface of the light transmitting layer.
In the thickness detection method of the present invention, in which the thickness of the light transmitting layer is detected from the focussing error signals, the thickness of the light transmitting layer can be detected without requiring dedicated detection means.
According to the present invention, the spherical aberration ascribable to the thickness error of the light transmitting layer can be suppressed even if the numerical aperture NA of the objective lens increased. Therefore, the information recording medium can be improved in recording density and in recording capacity without raising the production cost of the information recording medium while the tolerance of the thickness error of the light transmitting layer is maintained at a larger value.